1. Field of the Invention
The present invention relates to catalysts and methods for preparing the catalysts and, more particularly, to a catalyst for producing hydrogen and a method for preparing the same.
2. Description of Related Art
Energy is indispensable in our daily life. About 80% of energy relies on combustion of fossil fuels, so the generated carbon dioxide and greenhouse gases result in global warming. To solve the issue of environmental pollution, it is urgent to find a renewable, clean and sustainable new energy to replace the fossil fuels.
Hydrogen is an excellent energy carrier because it is an alternative energy with high electrical energy conversion efficiency. Heat quantity for combustion of per kilogram of hydrogen is about 3 times of that of gasoline and 4.5 times of that of coke. The product of reacting hydrogen with oxygen is produced in the form of water and produces low-pollution for the environment. However, a volume energy density of hydrogen is low. To increase the energy density per unit volume, biofuels can be used to produce hydrogen. Furthermore, expensive freight of hydrogen leads scientists to choose appropriate hydrogen sources as fuels of the new energy sources. At present, alternative hydrogen sources including hydrocarbon such as methanol, ethanol, natural gas and light oil are used. Among these, ethanol has many advantages such as higher fuel quality, cheap price, easy access, easy storage, easy portability and higher energy density. Furthermore, ethanol can produce hydrogen at lower reaction temperature. As compared to the conventional gasoline, the generated carbon dioxide is reduced by approximately 50% and the air pollutant such as nitrogen oxide, sulfur oxide and hydrocarbon will not be produced.
Current technologies for converting ethanol into hydrogen include steam reforming of ethanol (SRE), partial oxidation of ethanol (POE), oxidative steam reforming of ethanol (OSRE) and others. Over the past decade, scientists have focused on the research that SRE can be operated at lower temperature. The chemical reaction equation for SRE is as follows.C2H5OH+3H2O→2CO2+6H2 ΔH0298=+347.4 kJ/mol
The SRE reaction belongs to a reaction having highest yield of hydrogen. However, since SRE is an endothermic reaction, the operation temperature is still high. In contrast, OSRE is an exothermic reaction and can react at relatively lower temperature, so it has become the research emphasis in the industry. The chemical reaction equation for OSRE is as follows.C2H5OH+½O2+2H2O→2CO2+5H2 ΔHR=−68 kJ/mol
According to previous studies, it is known that metals are used as a catalyst for the catalytic reaction of hydrogen. The most commonly used catalysts are used by placing noble metals with high activities on the oxides carriers. The noble metals include rhodium (Rh), ruthenium (Ru), platinum (Pt), palladium (Pd), iridium (Ir) and the like. The oxides carriers include aluminum oxide (Al2O3), magnesium oxide (MgO), lanthanum oxide (La2O3), silicon dioxide (SiO2) and the like. However, the temperature of the catalytic conversion is mostly higher than 500° C. which increases the fracture between carbon and carbon as well as generate byproducts deposited on the catalyst surface. Thus, loss of the catalytic activity requires higher production costs.
Schmidt et al., in Science, 2004, 303, 993-997, have reported a steam autothermal reforming reaction of ethanol. This study has proposed that 5% of Rh—CeO2—Al2O3 was used as catalyst for converting ethanol into hydrogen and also used a two-stage catalytic experiment. In the second stage, platinum-cerium dioxide (Pt—CeO2) was used as catalyst. However, the contents of rhodium and platinum are too high, and precious rhodium and platinum will increase production costs. The reaction temperature of the two-stage catalytic experiment being over 400° C. also results in the byproducts deposited on the catalyst surface and the activity loss of the catalyst. In addition, Andonova et al., in Applied Catalysis B: Environmental, 2011, 105, 346-360, indicated that when cobalt was added to nickel-aluminum oxide (Ni—Al2O3), and the hydrogen selection ratio was increased in the bimetallic effect at the optimal content of 6 wt % of cobalt and 10 wt % of nickel. Although the use of nickel can reduce the cost of the catalyst, such catalyst was demanded to react at high temperature of 500° C. The high temperature will increase the fracture between carbons and generate byproducts such as CO2, C2H4 and CH3CHO deposited on the catalyst surface and lose the catalyst activity. Thus, the catalysts proposed in the studies do not meet the market demand.
Therefore, it is urgent to develop a catalyst whose catalyst activity can be maintained under the condition of the lower temperature of the ethanol oxidation reformation, solve the situation of the carbon deposition, and reduce the content of noble metal in the catalyst for reducing the production costs.